1. Field of the Invention
The present invention relates to an optical tap used to select signals transmitted by wavelength division multiplexing in light transmissions and in light LAN (local area network) communications.
2. Related Art of the Invention
In recent years, as computer technology has evolved, LANs (local area networks) have spread as data communications among terminals. Today, time sharing is used to transmit and exchange data among terminals. As networks have been improved in performance and made more versatile, it has been necessary to treat images, voices, and other information as well as data. To transmit these kinds of information, the transmission capacity must exceed 100 M bps. However, limitations are imposed on the transmitting function of the current time-sharing LANs. Therefore, there is a demand for LANs of higher speeds and higher throughputs.
In optical fiber communications, wavelength division multiplexing (WDM) is available to transmit multichannel signals over a single fiber. In the transmitter, signals of various channels are wavelength division-multiplexed with light signals of different wavelengths. The wavelength division-multiplexed signals are transmitted to a receiver via a single fiber. The receiver selects and demodulates a desired light signal by an optical separator or optical tuner. Instead of the use of such a optical separator or optical tuner, only a desired wavelength is selected, and other light signals are passed as they are. In this way, a LAN which has exchanging function, operates at a high speed, and excellent in economy can be built (Technical Research Report OQE-91-126, the Electronic Information Communications Society, p. 61, 1992).
An optical tap is used to select signals as described above. This optical tap is composed of two light-polarizing beam splitters and a wavelength-selecting light polarization converter utilizing an acousto-optic effect. Light signals of a channel to be selected are polarized by the wavelength-selecting light polarization converter. Only the desired light signal is passed through the beam splitters to take an output light.
However, the direction of polarization of light transmitted over ordinary fibers is not definite. In addition, the direction is varied by the external environment. Therefore, the plane of polarization of light incident on the optical tap jitters. As a result, the intensity of the output light fluctuates, and noise is induced. The above problems may be solved by using a plane of polarization-maintaining fiber as the transmission fiber. However, this is more expensive than ordinary fibers. Also, it is more difficult to connect the plane of polarization-maintaining fiber. Furthermore, the application of this fiber is limited, since currently widespread fiber paths are not formed by plane of polarization-maintaining fibers.
Additionally, the wavelength-selecting light polarization converter itself results in jitters because of its temperature dependence and power variations. There exist optical separators and optical tuners capable of selecting wavelengths, but they select only certain wavelengths. Signals of wavelengths not selected are discarded. That is, they are unable to make light signals branch off. Consequently, they cannot be used as optical taps.